1. Field of the Invention
This specification relates to a method for calculating a spacing ratio of an interferometer array antenna which finds a direction through phase comparison.
2. Background of the Invention
In general, a direction finder is a receiving apparatus which measures a direction that electric waves are coming, and also referred to as a radio direction finder. The direction finder does not emit electric waves (radio waves) unlike radar. The direction finder is used for safety of ships and aircrafts and for identifying or monitoring unidentified or unlawful radio waves.
The direction finder includes an antenna for receiving a radio wave and a receiver for processing the received radio wave. The antenna and the receiver of the direction finder have various structures and shapes according to a way of finding a direction.
Examples of various direction finding techniques include spinning direction finding, amplitude comparison direction finding, phase comparison direction finding which is so-called an interferometer, and the like.
The interferometry is a technique for finding a direction using a phase comparison method. This is a technique used for high-accuracy location of emitters operating at frequencies from just above direct current (DC) to well above light. The interferometer may include two matched antennas and two matched receivers. Radio waves (radio signals) which are received through antennas of two channels in the interferometer are converted into intermediate frequencies by the respective receivers so as to be passed to a phase comparator, which measures a relative phase angle according to directions that the two signals are arriving. The relative phase angle is then passed to a direction finding processor, namely, a processor, which then calculates an angle of arrival (AOA) of the radio waves relative to a baseline which is the orientation of the two antennas. In a general interferometric system, the processor accepts information relating to the orientation of the baseline (true north or true local horizontal), to determine the true azimuth or elevation angle to the emitter.
An interferometer array antenna according to the present disclosure is an antenna system having two or more antennas, and may determine an azimuth of a received signal by using a state phase difference which is measured between the antennas. Especially, in order to estimate an AOA of a radio signal source including elements of an electronic war field, such as early warning threat detection, tracking, and electronic information, phases of the radio signal sources are compared to measure an azimuth of the radio signal source. When an array spacing is acquired for obtaining high accuracy, a spacing between antennas is one of important factors for avoiding ambiguity and satisfying required accuracy of direction finding.
The antenna spacing may be acquired by a probability calculation which minimizes ambiguity in consideration of electrical phase errors due to a signal-to-noise ratio and errors due to electrical phase non-matching of elements, or by using disjoint sets to avoid a phase difference set output by the array antenna from being repetitively generated in an instantaneous field of view (IFOV).
The first method requires complicated processes and the second method is simple but has a disadvantage that phase difference shift which is generated in a direction finder is not considered.
Therefore, a method for obtaining (calculating) an array spacing of a is direction finder by making up for those methods is required.